1. Field of the Invention
This invention relates to an engine control device and method for controlling a ratio of air contained in fuel that is supplied to an engine (air-fuel ratio) based on a measurement result of oxygen density in an exhaust gas from the engine.
2. Description of Related Art
Recently, to reduce harmful contents in exhaust gas and obtain an optimum combustion state, a feedback-control-type engine control device has been proposed that converges an air-fuel ratio inside the engine to a theoretical air-fuel ratio (stoichimetric control). The device detects the density of oxygen in exhaust gas using an oxygen sensor mounted on the exhaust system, and controls the supply quantity of fuel or air based on a correction quantity in response to the detected oxygen density, thus adjusting the air-fuel ratio of the intake air-fuel mixture to a proper air-fuel ratio.
Some such feedback-control-type devices adopt an oxygen feedback-control method such as that shown in FIG. 5. When the air-fuel ratio of the air-fuel mixture in the exhaust gas is shifted to a rich side, as in range “a”, the oxygen feedback control reduces the correction quantity to approximate the air-fuel ratio to the theoretical air-fuel ratio. At the same time, when the air-fuel ratio of the air-fuel mixture in the exhaust gas is shifted to a lean side, as in range “b”, the oxygen feedback control increases the correction quantity to approximate the air-fuel ratio to the theoretical air-fuel ratio.
In such a prior art engine control device, when oxygen feed back control is performed in a predetermined operation state, such as during acceleration time, engine output tends to be lowered. Accordingly, as shown in FIG. 5, at the time of acceleration, the oxygen feedback control is temporarily stopped by setting the correction quantity to zero, and only a usual control that uses a control map preliminarily stored in the engine control device is performed. However, in this prior art method of oxygen feedback control, there is a drawback in that, at the time of acceleration, the correction quantity is rapidly changed as shown in FIG. 5. This causes the fuel injection quantity to become unstable, which makes it difficult to approximate the intake air-fuel mixture to a target air-fuel ratio.